Compression bone screws are commonly used to join together, under compression, fractured fragments of a broken bone, and are inserted during a surgical intervention to precisely locate the screw in a desired location straddling the fracture site within the bone. In general, a compression screw allows the fractured bone to increase in strength and favours prompt healing, while decreasing the risk of non-union.
The scaphoid bone, located in the wrist generally between the radius and thumb, is the mechanical link between the proximal and distal carpal rows, is one of the most commonly fractured bone in the upper extremity. Wrist fractures often involve the scaphoid bone, and primarily occur due to axial compression with the wrist in extension, inducing a flexion moment on the scaphoid and causing a fracture. As a result of the flexion moment applied, as well as its relative small size and its location, displacement and/or deformity of the scaphoid is common. In addition, the scaphoid has a limited blood supply, and thus can become avascular after a fracture, which may lead to bone necrosis.
Consequently, it is often necessary to surgically intervene to treat a fracture of the scaphoid. Such surgical intervention typically involves the insertion of a compression bone screw in the scaphoid. Although un-displaced fractures have a high rate of healing when the wrist and thumb are immobilized for a prolonged period of time, there has nonetheless been a growing trend toward surgical intervention for fixation of acute un-displaced fractures (i.e. bone fractures wherein cracks in the bone may radiate in several directions but the bone fragments do not separate) of the scaphoid. However, most scaphoid fractures are displaced, and surgical intervention is clearly warranted in these cases. Surgical intervention and rigid internal fixation have shown to decrease time of healing and risks of non-union. Surgical intervention requires precise positioning of the implant or fracture fixation structure along the central axis of the scaphoid and spanning the fracture site, rigid fixation under compression and limited disturbance of the biology of the fracture site.
Due to the small size of the scaphoid bone (typically about 22 to about 30 mm in length), many challenges exist for designing and manufacturing bone screws suitable for use in the fixation of the scaphoid. Currently, compression screws used in scaphoid fixation have been designed to be left in place permanently, buried under articular cartilage and improvements have been made to the screw to increase compression. A commonly employed scaphoid fixation screw includes screw threads on opposed ends of the screw, each having a different pitch, such as to hold the portions of the scaphoid on opposite sides of the fracture site in compressive engagement with each other.
However, accurate placement of the screw and minimization of the space occupied by the screw such as to increase healing potential, are examples of areas where improvement is sought. Particularly when used for fracture fixation of very small bones such as the scaphoid, existing bone screws have been found to occupy valuable space where bone cannot re-grow and therefore the actual amount of bone available for healing of the fracture is often not optimized.
Accordingly, there remains a need for an improved screw for fracture fixation of bones, particularly but not necessarily small bones such as the scaphoid, which will permit both accurate placement of the screw within the bone and which optimizes the amount of bone available for healing of the fracture.